1. Field of the Invention
This invention relates generally to optical imaging characterization, and more particularly, to a method and apparatus for improving the signal to noise ratio of optical imaging for use in the characterization of imaging systems used in the semiconductor industry.
2. Discussion of the Related Art
FIG. 1 illustrates a prior art optical stepper lithography apparatus 20. In typical semiconductor chip processing configuration, a pulsed-laser signal source 22 provides a signal made up of a plurality of successive pulses at a given pulse frequency or repetition rate R. The pulses are provided to a lithographic imaging system 24 including lenses 26, 28 imaging a reticle positioned at the station 29 onto a wafer 32 on a stage 34. Also mounted on the stage 34 is a slit plate 36 which contains several slits 38, each oriented in a different direction, and a photodetector 40 beneath the slit plate 36.
In the use of the apparatus 20 of FIG. 1, and also with reference to FIG. 2, the apparatus 20 is used for measuring optical performance of a stepper. The reticle is replaced with a test reticle 30 is positioned at the station 29. The stage 34 is positioned as shown in FIG. 1 with the test reticle 30 imaged on the slit plate 36. The slit 38 is then moved or scanned across the image plane, the longitudinal axis of the slit 38 being held perpendicular to the scan direction. During the scanning operation, the intensity of the transmission through the slit 38 is monitored by means of photodetector 40, which, with the slit plate 36, makes up an aerial image monitor 42. The intensity profile measured by the photodetector 40 is provided to a data output recorder 44 and is compared to a previously calculated profile expected from a xe2x80x9cperfectxe2x80x9d stepper or to a baseline measurement. From this comparison the performance characteristics of the apparatus 20 are determined.
As the dimensions of features being printed with lithography systems get smaller and smaller, the requirements on the precision of aerial image monitors become more and more severe, and the available source power tends to decrease, leading to smaller signal levels. High precision can only be achieved with a high signal to noise ratio, i.e., a high ratio of the pulsed-laser signal detected intensity to the intensity of the noise that is detected, so that small changes in the signal corresponding to small variations in the intensity of the aerial image can be recorded
Again referring to FIG. 2, as noted above, the signal 46 from the pulsed-laser source 22 is provided as a series of pulses 48 at a given repetition rate R. Because each pulse 48 tends to have a slightly different intensity, the laser output signal exhibits noise over a wide frequency range.
A typical general noise frequency spectrum (signal intensity I plotted against frequency) of the signal 46 provided by the pulsed-laser source 22 is shown in FIG. 3, indicated by the sloping, irregular line 50. The vertical line 52 indicates the laser pulse rate (or frequency) R.
FIG. 4(a) illustrates the aerial image 53 produced by the imaging system, and FIG. 4(b) shows the signal 56 produced by the photodetector 40, which is recorded by the data output recorder 44. This signal from the photodetector 40 includes a substantial amount of noise, as shown in FIG. 4(b), which is a graph of signal 56 intensity provided by the photodector 40 to the data output recorder 44 plotted against time.
FIG. 4(c) shows an enlarged view of a short segment of the signal of FIG. 4(b), where individual laser pulses can be distinguished.
With such a poor signal to noise ratio, high precision of the aerial image monitor 42 cannot be achieved.
Therefore, what is needed is a method and apparatus for improving the signal to noise ratio of aerial image monitors for lithography stepper tools used to produce semiconductor devices.
In the present method for monitoring the image of a stepper imaging system, a pulsed-laser signal is provided, and is modulated at a chosen frequency. The modulated signal it is then provided to an imaging system. An output signal is provided from an aerial image monitor in response to the modulated signal provided to the optical imaging system, and only the component of that signal at the chosen frequency is amplified.
The present image monitoring apparatus includes a signal modulating device in the form of an optical chopper for modulating a laser pulse signal at a chosen frequency. Image monitoring means receive the modulated signal and provide an output signal in response to the modulated signal. A lock-in amplifier amplifies the component of the output signal at the chosen frequency.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described an embodiment of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.